Add Sentry.add_global_event_processor API

[sl0thentr0py]

Other SDKs allow adding global event processors without configuring scope, so this adds the same functionality.

closes #1974
(need it to implement #1975)

[codecov-commenter]

Codecov Report

Base: 98.61% // Head: 98.61% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (9c16bb2) compared to base (fb394a5).
Patch coverage: 96.96% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1976   +/-   ##
=======================================
  Coverage   98.61%   98.61%           
=======================================
  Files         157      157           
  Lines        9880     9911   +31     
=======================================
+ Hits         9743     9774   +31     
  Misses        137      137           

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry-ruby.rb	95.88% <50.00%> (-0.55%)	⬇️
sentry-ruby/lib/sentry/scope.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry/scope_spec.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/breadcrumb.rb	100.00% <0.00%> (+3.70%)	⬆️

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[st0012]

Generally looks good 👍 But it'd be better if we don't use class variable for it.